With 70,000 vehicles in its nationwide fleet, AT&T’s cars, vans, and trucks leave quite the carbon footprint — not the mention quite the gas bill. But thanks to a new deal with General Motors, both of those should shrink a little.

That’s because the giant telco recently purchased 1,200 new compressed natural gas (CNG)-powered Chevrolet Express vans, apparently the largest CNG vehicle order GM has ever received.

The van purchase is part of AT&T’s broader 10-year plan to replace more than 15,000 of its vehicles with alternative-fuel models by 2018. Part of that plan is a five-year, $350 million effort to buy 8,000 CNG vehicles, which will complement other rollouts of electric and hybrid vehicles. In January the company announced that its 5,000th alternative-fuel vehicle had hit the road — and naturally, it was a Chevy Express van.

Thus far, AT&T seems to have taken on an active role in moving toward a greener fleet. In a statement to the SmartVan, AT&T said that according to the U.S. EPA, CNG vehicles produce 25 percent fewer carbon dioxide emissions than petroleum-burning models. The company also said that while it’s too early to tell how much fuel the new vans will save, the company’s initial CNG vehicle rollout, in 2010, “helped the company avoid the purchase of more than one million gallons of traditional petroleum fuel.”

AT&T is also part of a U.S. Department of Energy program called the National Clean Fleets Partnership, in which big companies pledge to reduce their petroleum use. And CNG vehicles are likely to play a big role in that. Recent findings from Pike Research forecast a 68 percent increase in global CNG vehicle sales by 2016.